The development of downhole instrumentation to evaluate drilling and coring of earth formations, has been given impetus by various governmental committees and councils. Prognosis: While instrumentation and uses involving measurements while drilling (or "MWD"), are well-documented, gains to be obtained from measurements while coring (or "MWC") have not yet crystallized. Reasons: Many of most difficult well control problems occur when a core barrel is the well bore. Not only is the ability to handle well kicks reduced (because of reduced circulation capability) but there is increased likelihood of plugging and jamming.
That is to say, the benefits to be gained from MWC during exploratory coring have not been documented in sufficient fashion to outweigh the safety concerns of the field operators. Moreover, the type of real-time data desired or justified, is subject to speculation.
In our prior applications, op. cit., we describe use of a single measurement means mounted adjacent to the uphole terminus of the inner core barrel to monitor rotation of the inner barrel during coring operations. Rotation is determined by any change in repetition interval between uphole recorded mud pulses produced by a mud pulse system at the MWC drilling string segment uphole from the Hall-effect device.
During tripping down of the modified core barrel to the core site, rotation of the inner barrel could occur. It would be beneficial to provide means for releasably locking the inner barrel relative to and adjacent to the imbedded Hall-effect, under such circumstances, so as to facilitate initial testing of the Hall-effect device (to assure operability).